1. Field of the Invention
The present invention relates to radiation detectors and, more particularly, but not by way of limitation, to a novel entrance window for gas filled radiation detectors.
2. Background Art
While the present invention is described with reference to gas filled proportional radiation detectors, it is applicable as well to any gas filled radiation detectors.
Large area gas filled proportional detectors are used extensively in health physics for surface contamination detection, particularly for detection of radioactive contamination of personnel. Sealed proportional detectors are preferable in practical applications because they do not require constant gas supply and the associated periodical replacement of gas bottles. This not only reduces the operating costs, but also minimizes the possible out of service time related to the gas bottle replacement.
The efficiency of a detector is a critical parameter that affects important properties of a contamination detection instrument: Minimum Detectable Activity (MDA) and monitoring time (which is especially critical in case of monitoring people). The higher the efficiency of a detector, the lower the MDA and the shorter the monitoring time. The detector efficiency directly depends on the radiation absorption by the entry window. This absorption in turn depends on the surface density of a window material. Practical upper limit of the window thickness for beta sensitive detectors in terms of overall surface density is about 5 mg/cm2.
Only three light metals can be considered as window materials: beryllium (Be), aluminum (Al) and titanium (Ti). Utilization of the beryllium windows in large area detectors (larger then 100 cm2) is practically impossible because of its high cost. Aluminum with its specific density of 2.7 g/cm3 allows manufacturing of inexpensive foils with the surface density of 5 mg/cm2 and the thickness of 18.5 μm. Unfortunately, Al is known as a very porous material and there is high probability of the presence of pinholes in such a thin foil. Finally, titanium has a specific density of 4.5 g/cm3. Cold rolling technology allows manufacturing of relatively inexpensive foils of thickness down to 10 μm, corresponding to a surface density of 4.5 mg/cm2. The manufacturing process leads however to the inherent defects like micro cracks that have the tendency to migrate and even to develop to pinholes under the influence of a mechanical stress. It is a stochastic process and may take unpredictable time (from days to years). Certainly this is a very undesirable effect from the point of view of the reliability and the lifespan of a detector.
Obviously, the operating lifetime of a detector is a very critical parameter. In the case of gas proportional detectors there is practically no inherent limitation other then the gas leakage, or a gas filling contamination. It is known that one of the biggest practical problems with sealed proportional detectors is the leakage of a counting gas through detector windows. When thin metal foils are used as a window material, the leakage may become a problem due to the reasons outlined above. This affects the detector production yield and detector lifespan. Practically, the average lifespan is in the range of 12-24 months. It is also inconsistent from one detector to another due to the statistical spread of defects in window materials.
Donachie, Matthew, Titanium, A Technical Guide, ASM, 2000, and Tada, Hiroshi, The Street Analysis of Cracks, Handbook, ASM, 2000, discuss the technological problems with manufacturing this metal foils and inherent micro-defects in these foils that, over time and under mechanical stress, may eventually develop into bigger cracks and pinholes and lead to gas leakages and, consequently, to premature detector failures.
In flow detectors, metallized plastic materials have been used for years. They feature low surface density (even down to 0.4 mg/cm2) but also relatively high gas permeability. Such a technical solution is used, for example in U.S. Pat. No. 3,296,478, in which an entrance window is made from polycarbonate resin, less than 1 μm in thickness. This window can work only in flow detectors and is not useful for sealed counters. U.S. Pat. No. 5,345,083 describes an entrance detector window made from polypropylene or polyethylene terephthalate coated on the inner side with gold, platinum or iridium. This window also cannot work in sealed detectors and is suitable only for gas flow units.
In recent years, a number of so-called high barrier plastic materials have been developed. They feature very low gas permeability, especially when metal coated. The choice of the thickness is limited though. Still, there are a few materials available that meet in this regard the requirements of a detector window (thickness is the range 12-36 μm). Commercially available barrier foils do not have metal coatings sufficient for detector applications: the coating (if any) is usually on one side only and its thickness is insufficient to provide the required electrical conductivity and light tightness.
Accordingly, it is a principal object of the present invention to provide a radiation detector window for sealed radiation detectors that overcomes the known and described above problems with existing window materials.
It is a further object of the invention to provide such a radiation detector window for sealed detectors that limits the internal pressure drop to less than 10% in 5 years.
It is another object of the invention to provide such a radiation detector window that has a total surface density below about 5 mg/cm2.
It is an additional object of the invention to provide such a radiation detector window that has good electrical conductivity (surface resistivity less than about 1 Ohm/square).
Yet a further object of the invention is to provide such a radiation detector window that has good light tightness (optical density of at least about 4).
Yet an another object of the invention is to provide such a radiation detector window that has good adhesion of metal coatings to the plastic core.
Yet an additional object of the invention is to provide such a radiation detector window that has high quality of surface (no pinholes or other defects).
A further object of the invention is to provide such a radiation detector window good and stable mechanical properties.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.